Rett Syndrome: Genetic Treatment Approaches
Rett syndrome is a genetic disorder that occurs almost exclusively in females. Symptoms of Rett syndrome begin to surface around six months after birth (1). Rett syndrome is characterized by a number of symptoms including slowed growth, loss of normal coordination, inability to communicate, abnormal hand and eye movements, breathing problems, seizures, scoliosis and more (1). Individuals with the disorder can live to over 50 years of age, but require home care for much of their life. Rett syndrome is caused by mutations in the methyl CpG binding protein 2 gene (MECP2) on the X chromosome. Because the X-linked mutation is dominant, hemizygous males die of neonatal encephalopathy. Rett syndrome typically occurs (>90%) as the result of a sporadic somatic mutations (2). In rare cases a mutant allele of the gene can be passed from unaffected mothers to their offspring. MECP2 Mutation Effects Systematic genetic screens identified MeCP2 as the gene involved in Retts syndrome. The MeCP2 protein is classified as an epigenetic factor, and research has implicated it in a number of processes such as transcriptional regulation, RNA-binding protein, and general activator. While MeCP2 seems to play myriad roles in DNA/RNA and gene regulation, no underlying function has been found (3). In an effort to begin understanding the changes the disorder causes, Colantuoni et al used cDNA microarray technologies, subtractive hybridization, and and conventional biochemistry to generate gene expression profiles from postmortem Rett patients (3). Glial transcripts, which are known to be involved in nueropathological mechanisms, were upregulated, while many neuron specific mRNAs were found to be downregulated (3). The activation of glial cells shows the brain is trying to protect itself, and resources drawn from normal processes to elevating glial cell activity may reduce normal mRNA expression by the neurons. Gene Therapy Provides Hope Due to the complexity of MeCP2 function, finding traditional treatments for Rett syndrome has been exceedingly difficult. Rett syndrome was a suitable candidate for gene therapy because the target gene had been identified, but treatments to deal with the improper protein were difficult to find. While multiple groups have been working on gene therapy solutions to Rett syndrome, this article will focus on the most recent research by Garg et al. The research group used the Adeno-associated virus (AAV9) to create a vector with a functional version of the MECP2 gene. AAV9 was used for its ability to cross the blood brain barrier and also because it infects neuronal and non-neuronal cells in the brain. This is specifically important because glia cells, which are non-neuronal, are a key component to the disorder and reestablishment of MeCP2 just in glia have produced positive results (4). The major problem confronting the researchers was whether they could deliver MeCP2 cDNA via a viral vector at physiological levels. Lower levels would not be sufficient to rescue/recover and levels higher than normal physiological concentrations have been shown to be toxic (4). In order to answer this question the recombinant AAV9 virus with Cre recombinase (conditional knockout) was used to rescue Rett syndrome symptoms in mice (Figure 1). MeCP2 was expressed normally and within the context of its normal regulatory elements (4). cDNA was then cloned under the MECP promoter into the AAV9 vector, and delivered into MeCP null mice systematically to test levels. Both methods resulted in delivery of MeCP2 (at specific concentrations) to the brain and significant improvement or reversal of typical Rett syndrome symptoms in gender appropriate mice. Furthermore the MeCP2 protein that was made bound heterochromatin and restored normal neuron size (4). Rett syndrome has been exceptionally well characterized and therefore thoroughly researched. The gene of interest has been identified as well as it's protein product. Even the mechanism by which an individual acquires the affliction is known. For these reasons researchers have made great strides to finding a gene therapy treatment that could replace MeCP2 mutant allele by introducing the proper cDNA via an Adeno virus. Following this very successful research, the Rett Syndrome Research Trust has announced its first pre-clinical trial to study the reversal of Rett symptoms using gene therapy. The trial will begin sometime in the Fall/Winter of 2013. References 1. http://www.mayoclinic.com/health/rett-syndrome/DS00716 2. Amir RE, Van den Veyver IB, Wan M, Tran CQ, Francke U, Zoghbi HY. 1999. Rett syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2. Nature genetics 23: 185-8 3. Colantuoni C, Jeon OH, Hyder K, Chenchik A, Khimani AH, et al. 2001. Gene expression profiling in postmortem Rett Syndrome brain: differential gene expression and patient classification. Neurobiology of disease 8: 847-65 4. Garg SK, Lioy DT, Cheval H, McGann JC, Bissonnette JM, et al. 2013. Systemic delivery of MeCP2 rescues behavioral and cellular deficits in female mouse models of Rett syndrome. The Journal of neuroscience : the official journal of the Society for Neuroscience 33: 13612-20